题目：

给定一个数字列表，返回其所有可能的排列。

class Solution {
public:
    /**
     * @param nums: A list of integers.
     * @return: A list of permutations.
     */
    vector<vector<int> > permute(vector<int> nums) {
        // write your code here
        vector<vector<int> > result;
        if (nums.size() == 0 || nums.size() == 1) {
            result.push_back(nums);
            return result;
        }
        sort(nums.begin(), nums.end());
        while (true){
            int p, i;
            result.push_back(nums);
            for (i = nums.size()-1; i >= 0; i--) {
                if (i <= 0)
                    return result;
                if (nums[i-1] < nums[i]) {
                    p = i-1;
                    break;
                }
                }
            for (int j = nums.size()-1; j > p; j--) {
                if (nums[j] > nums[p]) {
                    swap(nums[p], nums[j]);
                    break;
                }
            }
            reverse(nums.begin() + p+1, nums.end());
        }
    }
};

9.全排列